Hearing clarity is taken for granted by most people; however, temporary periods of altered auditory perception are a normal part of existence. Nearly every individual will experience phantom noises such as “ringing in the ears” during their lifetime as a result of exposure to external and/or internal stimuli. Loud noise (reported by Zhang et al., Wallhausser-Franke et al.), certain pharmaceuticals, stress, and various physical conditions can all cause altered auditory perception, usually on a temporary basis. However, for some individuals with auditory dysfunction, altered auditory perception can be permanent.
Auditory dysfunctions are common. For example, in the United States, the prevalence of tinnitus when the whole population is considered is approximately 3%. This prevalence is only 1% under the age of 45 but increases significantly with age, rising to 9% in the population over 65 years (Adams et al., 1999). This roughly translates to 36 million Americans with tinnitus (Heller 2003). Tinnitus is a noise in the ears, often described as ringing, buzzing, roaring, or clicking. Subjective and objective forms of tinnitus exist, with objective tinnitus often caused by muscle contractions or other internal noise sources in the area proximal to auditory structures. In certain cases, external observers can hear the sound generated by the internal source of objective tinnitus. In subjective forms, tinnitus is audible only to the subject. Tinnitus varies in perceived amplitude, with some subjects reporting barely audible forms and others essentially deaf to external sounds and/or incapacitated by the intensity of the perceived noise.
Because auditory dysfunctions often occurs secondary to a pathological state, initial treatment may focus on finding an underlying cause. A subject presenting with, for example, tinnitus may be asked for information regarding medications, recent or chronic noise exposure, and home and work environment. Common medications such as aspirin are known to cause tinnitus in some patients or in elevated dosages. Stress can be a direct cause of tinnitus and can aggravate existing cases. A thorough physical exam is typically made of a subject with complaints of tinnitus to eliminate pathologies such as hypertension, tumors, and infections. Objective tinnitus may be diagnosed using a stethoscope if the source of the noise can be localized. For example, hypertension or arterial disorders may produce objective tinnitus, as the carotid arteries pass close to the auditory organs in humans, and excessive pressure or arterial blockage may cause detectable noise to both the subject and to an outside observer.
If a treatable underlying cause to the auditory dysfunction is identified, treatment may focus on elimination of the cause. For example, hypertensive patients may see a reduction or elimination of tinnitus once anti-hypertensive therapy begins. However, a significant number of patients have untreatable underlying pathologies or have tinnitus in the absence of any identifiable cause. For these patients, treatments for directly reducing or eliminating auditory dysfunction are desirable.
Tinnitus research is actively pursued in the hope of finding efficacious treatments. Recently published work has utilized drug delivery systems such as the system described in U.S. Pat. No. 5,713,847, which includes a catheter inserted into a patient's auditory cortex or thalamus for microinfusing drugs. U.S. Pat. No. 6,656,172 describes a tinnitus treatment that includes inserting intrathecally a catheter for infusing a drug. Other treatment methods may try to mask the perceived tinnitus noise by generating an audible signal of appropriate frequency. WO 01/08617 describes a system with a vibrating probe placed in proximity to the inner ear.
The use of certain stimulating electrodes to mask tinnitus has been published. U.S. Pat. Nos. 5,735,885 and 5,496,369 describe the placement of an electrode in the primary auditory cortex of a patient. U.S. Pat. Nos. 6,456,886 and 5,697,975 also use an electrode placed the auditory cortex, and further describe placement of an electrode in the medial geniculate body of the thalamus.